Embodiments of the subject invention pertain to detection of radiation related to, for example, nuclear nonproliferation activities. The Office of Nuclear Nonproliferation Research and Development has been focused on enabling the development of next generation technical capabilities for radiation detection of nuclear materials. Recently, the Special Nuclear Material (SNM) Movement Detection Program created an evolving technology roadmap that identified the following fundamental objectives:                1. detect shielded highly enriched Uranium;        2. detect SNM at standoff distances; and        3. detect shielded weapon-grade plutonium.        
General features of an advanced detection system able to meet the above objectives may include, but are not limited to the following:                1. a large area detector, e.g., at least several square meters;        2. capable of efficiently detecting and distinguishing fast neutrons, with gamma discrimination of at least 10,000:1 and preferably 100,000:1;        3. capable of efficiently detecting and distinguishing thermal neutrons, with gamma discrimination of at least 100,000:1 and preferably at least 1,000,000:1;        4. a nanosecond time resolution capability for neutron time of flight measurement, multiplicity measurements, as well as coincidence counting for active interrogation;        5. a segmented detector;        6. robust, moderate price, stable, safe, and easily field deployable.        
There has been a major effort towards making a large area, efficient, fast neutron detector with good gamma discrimination. A recent review of methods of neutron detection is provided by Runkle R. C. et al., Journal of Applied Physics 108. 111101. (2010). There are many methods under investigation, yet none have appeared to come very close to meeting the challenging requirements.
Organic crystals, liquid scintillator, and specially designed plastic scintillator have been studied, (PCT Patent Application PCT/US2012/045094, published Jan. 3, 2013 under publication number WO 2013/003802, and “Plastic Scintillators with Efficient Neutron/Gamma Pulse Shape Discrimination”, N. Zaitseva et al., Nuclear Instruments and Methods in Physics Research A 668(2012) 88-93). The different materials have been studied in small volume detectors and their gamma ray discrimination abilities are similar. The gamma ray discrimination is measured by the difference in scintillation pulse shape of interactions of neutron and gamma ray. This technique is known as Pulse Shape Discrimination (PSD). These types of material have been shown to have Figures of Merit (FOM) of 2.5 to 3.5, corresponding to gamma ray discrimination factors of about 1,000:1. In these small volume detector studies, the scintillation material has an efficient optical coupling to a Photo Multiplier Tube (PMT). In large area detectors, it is almost inevitable that loss of some scintillation light will result in a reduced gamma discrimination factor. Accordingly, it appears the gamma ray discrimination of these detectors is inadequate for effective detection of SNM. In general, the PSD method suffers from an inherent disadvantage; the pulse shape discrimination is effective only after a time delay, at which point the pulse height has fallen by at least an order of magnitude. As a result, there is a major loss of quantum statistical information when using PSD, which limits the ability to provide powerful gamma ray discrimination in large detectors.
Pacific Northwest National Laboratory (PNNL-15214) has evaluated the performance of a large area (0.7 m2) twin sheet plastic scintillator time of flight (TOF) system for direct detection of fast neutrons. The TOF method yielded a gamma ray discrimination of at least 10,000:1, but only when the fast neutron intrinsic detection efficiency was limited to less than 4%. That detection efficiency is comparable to traditional moderator-based fast neutron detection systems (which have an intrinsic detection efficiency of 5 to 10% depending on the number of thermal neutron detectors), such as presently deployed in Radiation Portal Monitors (RPMs). This implies the TOF method would likely offer no improvement in the gross neutron counting sensitivity beyond that of existing RPMs. In addition, the intrinsic detection efficiency of up to 4% is inadequate to perform fast neutron multiplicity measurements, which are powerful indicators of SNM material. With the fast neutron multiplicity measurement technique, the measured count rate of two simultaneous fission neutrons is proportional to the square of the intrinsic detection efficiency. Thus, there is a premium on having intrinsic detection efficiency much higher than 4%. There is, therefore, a continuing need for fast neutron detection with both high intrinsic detection efficiency (>4%) and high gamma ray discrimination factor (>10,000:1 and preferably >100,000:1).
Similarly, there is a continuing need for thermal neutron detection with both high intrinsic detection efficiency (e.g., >5% and preferably >10%) and high gamma ray discrimination factor (e.g., >100,000:1 and preferably >1,000,000:1). To achieve high detection efficiency it is preferred to have a high concentration of a suitable isotope, such as Li-6 dissolved in the material. However, lithium forms polar compounds that are very poorly soluable (about (0.01% wt/wt) in non-polar scintillating plastics, such as polystyrene (PS) and polyvinyltoluene (PVT). Accordingly, a highly efficient, thermal neutron plastic scintillation detector has not yet been achieved.
Therefore, despite major efforts by Federal Agencies, National Laboratories and many researchers, there has been less progress than desired in meeting the fundamental objectives of improved neutron detection for the SNM Movement Detection Program. Accordingly, there is a need for a large area, fast and thermal neutron detector with high intrinsic efficiency and acceptable gamma discrimination.